1) Field of the Invention
The present invention relates to screws and drivers therefor, and particularly woodscrews having a countersunk head.
2) Description of Related Art
Traditionally, countersunk woodscrews are provided with a straight slot drive recess, but various forms of crossed slots, culminating in Philips® and Posidriv® forms of drive recess, have been developed.
However, whatever form of driver/recess is employed, an axial pressure is almost invariably required (and a pressure, moreover, that increases with increasing torque applied) in order to counter the tendency of drivers to “cam-out” of engagement with the screw recess.
Furthermore, as screws get larger it is usually necessary to increase the dimensions of the recess so that greater torque can be transmitted. However, this usually involves a different and bigger driving tool.
FR-A-2585609 discloses a drive system comprising a driving and a driven element each having a circular end face with upstanding sectors the edges of which are undercut. Consequently, when the driving and driven element are engaged face to face with their respective sectors interdigitating, the undercut edges transmit rotary drive between them with no cam-out. However, forming undercut edges on the head of a screw is problematic.
SU-A-438814 discloses a screw with a triangular recess, and a driver therefor. The recess appears to be undercut so that, after the driver has been inserted and turned through a small angle, undercut ledges on the driver engage with the recess undercuts to prevent disengagement. Again, forming undercut edges is problematic.